According to the recent requirement for automobiles to be fuel-efficient, conjugated diene-based copolymers, which have low rolling resistance, excellent tensile properties, and handling stability represented as wet skid resistance, are required as a rubber material for tires.
To reduce the rolling resistance of tires, there is a method of decreasing the hysteresis loss of vulcanized rubber. As evaluation standards of such vulcanized rubber, repulsive elasticity at 50° C. to 80° C., tan δ, Goodrich heating, and the like are used. That is, rubber materials with high repulsive elasticity at the above-described temperature range or low tan δ and Goodrich heating are preferably used.
As rubber materials having a low hysteresis loss, natural rubber, polyisoprene rubber, polybutadiene rubber, and the like are known, but these materials have low wet skid resistance. Thus, conjugated diene-based polymers or copolymers such as styrene-butadiene rubber (SBR) or butadiene rubber (BR), prepared by emulsion polymerization or solution polymerization, have recently been used as rubber for tires. Among these, the biggest advantage of solution polymerization over emulsion polymerization is that the contents of a vinyl structure and styrene, which determine physical properties of rubber, may be arbitrarily adjusted, and molecular weights, physical properties, and the like may be adjusted by coupling, modification, or the like. Thus, SBR prepared by solution polymerization through which it is easy to structurally change the finally prepared SBR or BR, movement of chain ends may be decreased by binding or modifying the chain ends, and the SBR may have increased binding strength with a filler such as silica, carbon black, or the like is widely being used as a rubber material for tires.
When such solution-polymerized SBR is used as a rubber material for tires, a glass transition temperature of rubber may be increased by increasing a vinyl content in the SBR, and thus physical characteristics required for tires, such as driving resistance and braking force, may be adjusted, and fuel consumption may also be reduced by appropriately adjusting the glass transition temperature. The solution-polymerized SBR is prepared using an anionic polymerization initiator, and chain ends of the formed polymer are bonded using various modifiers or are modified. However, when a reaction or coupling does not properly occur between the chain ends of the formed polymer and a modifier to be reacted or coupled therewith, an enhancement effect of physical properties required for tires, to be achieved due to the modified portion, is insufficient. For example, Japanese Patent Laid-Open Publication No. 2013-129693 discloses a method of preparing a modified conjugated diene-based polymer in which a reaction between a conjugated diene-based polymer and a modifier having an alkoxysilyl group occurs while stirring at a stirring power of 1 kW/m3 to 50 kW/m3, to enhance a modification rate. However, according to the preparation method, the modification rate of the conjugated diene-based polymer is still insufficient, and a molecular weight distribution of the modified conjugated diene-based polymer is also widened, and, accordingly, driving resistance is increased.
Therefore, there is a need to continuously conduct studies on methods of preventing physical properties of a conjugated diene-based polymer itself from deteriorating and efficiently enhancing an enhancement effect of the physical properties, derived from a modifier, in a reaction between the conjugated diene-based polymer and the modifier to prepare a modified conjugated diene-based polymer.